A laser is a device which has the ability to produce monochromatic, coherent light through the stimulated emission of photons from atoms, molecules or ions of an active gain medium which have typically been excited from a ground state to a higher energy level by an input of energy. Such a device contains an optical cavity or resonator which is defined by highly reflecting surfaces which form a closed round trip path for light. The active gain medium is contained within the optical cavity.
If a population inversion is created by excitation of the active medium, the spontaneous emission of a photon from an excited atom, molecule or ion undergoing transition to a lower energy state can stimulate the emission of photons of substantially identical energy from other excited atoms, molecules or ions. As a consequence, the initial photon creates a cascade of photons between the reflecting surfaces of the optical cavity which are of substantially identical energy and exactly in phase. This multiplication effect causes light inside the cavity to undergo gain, which, along with the feedback provided by the resonator, constitutes a laser oscillator. A portion of this cascade of photons is then discharged out of the optical cavity, for example, by transmission through one or more of the reflecting surfaces of the cavity. These discharged photons constitute the laser output.
Excitation of the active medium of a laser can be accomplished by a variety of methods. However, the most common methods are optical pumping, use of an electrical discharge, and the passage of an electric current through the p-n junction of a semiconductor laser.
Semiconductor lasers contain a p-n junction which forms a diode, and this junction functions as the active medium of the laser. Such devices, which are also referred to as diode lasers, are typically constructed from materials such as gallium arsenide and aluminum gallium arsenide alloys. The efficiency of such lasers in converting electrical power to output radiation is relatively high and, for example, can be in excess of 40 percent.
The use of flashlamps, light-emitting diodes and laser diodes to optically pump or excite a solid lasant material is well-known. Lasant materials commonly used in such solid state lasers include crystalline or glassy host materials into which an active material, such as trivalent neodymium ions, is incorporated. By way of example, when neodymium-doped Y.sub.3 Al.sub.5 O.sub.12, referred to as YAG, is employed as the lasant material in an optically pumped solid state laser, it can be pumped by absorption of light having a wavelength of about 808 nm and can emit light having a wavelength of 1064 nm.
It is possible to operate solid state lasers to produce temporally short pulses using the techniques of Q-switching or mode locking. The term "Q" refers to the ratio of energy stored in a resonant cavity to the energy loss per cavity round trip. In Q-switching, a controllable loss-producing device (the "Q-switch") is inserted into the laser cavity which inhibits laser oscillation. When the Q-switch is switched to its low-loss mode, the laser is suddenly able to oscillate, and a substantial fraction of the stored energy of the gain medium is released in a very short time, producing very high peak powers. Q-switches may be based on acousto-optical, electro-optical, or magneto-optical effects, or may be mechanical.
In the related technique of mode locking, a modulator, typically an acousto-optical device, is placed in the cavity to modulate the round-trip phases or amplitude of a laser cavity at frequency equal to the inverse of the cavity round-trip time. This has the effect of causing the longitudinal modes of the laser to phase lock, and forcing the laser output to have the form of a train of very short pulses, whose individual widths are of the order of the inverse of the gain bandwidth of the gain medium, and whose repetition rate is the inverse of the cavity round-trip time. It is also possible to achieve mode locking by a variety of techniques wherein the cavity loss or effective output coupling is caused to be a nonlinear function of the circulating power.
Lasers, particularly solid state lasers, operable at room temperature and capable of generating optical radiation in the eye-safe wavelength band above about 2 microns, are highly desirable for a number of important military and civilian applications.
Such military applications include use as rangefinders, target designators, and battlefield simulators. Civilian applications of eye-safe lasers include laser surgery and laser radars for wind and turbulence sensing. Most of these applications require short-pulse (.ltoreq.10 ns) operation, such as produced by Q-switched or mode-locked lasers, as described above. However, there are a number of problems with existing eye-safe solid state lasers in generating short pulses at room temperature.
The known commercially available existing eye-safe solid state lasers utilize either optically pumped host crystals doped with thulium (Tm) emitting near 2.0 microns or Tm-sensitized holmium (Ho) emitting near 2.1 microns. Good continuous wave (cw) performance with high optical-to-optical conversion efficiency has been achieved for Tm and co-doped Tm,Ho lasers because each absorbed pump photon results in two ions (rather than one) with an electron in the upper laser level. However, the short pulse, Q-switched performance of these lasers is inadequate.